multi: replace usage of fastsha256 with crypto/sha256

This commit removes all instances of the fastsha256 library and
replaces it with the sha256 library in the standard library. This
change should see a number of performance improvements as the standard
library has highly optimized assembly instructions with use vectorized
instructions as the platform supports.

channeldb/invoice_test.go

@@ -2,11 +2,11 @@ package channeldb
 
 import (
 	"crypto/rand"
+	"crypto/sha256"
 	"reflect"
 	"testing"
 	"time"
 
-	"github.com/btcsuite/fastsha256"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/roasbeef/btcutil"
 )
@@ -58,7 +58,7 @@ func TestInvoiceWorkflow(t *testing.T) {
 	// Attempt to retrieve the invoice which was just added to the
 	// database. It should be found, and the invoice returned should be
 	// identical to the one created above.
-	paymentHash := fastsha256.Sum256(fakeInvoice.Terms.PaymentPreimage[:])
+	paymentHash := sha256.Sum256(fakeInvoice.Terms.PaymentPreimage[:])
 	dbInvoice, err := db.LookupInvoice(paymentHash)
 	if err != nil {
 		t.Fatalf("unable to find invoice: %v", err)